Make: Projects

555 Timer Ball Whacker

This desktop demonstration circuit produces eerily lifelike behavior. Watch as a wooden arm swats at objects whenever they draw near.

555 ball whacker

This project uses a simple 555 timer chip and a feedback loop to control a servo-controlled wooden arm. Whenever an object comes close to a photosensor mounted on the end of the arm, it blocks the amount of light detected, which triggers the arm to swat the object away.

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Step #1: Create the Stand

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  • I designed a simple frame to hold all the parts, cut from 1/4" birch plywood.
  • For this design, you can use a 4" carriage bolt across the top, from which gold wire and thread can be tied to suspend the "ball," in this case, a plastic egg.
  • Here's a simple template that can be used to cut out your own stand. Feel free to tweak it to your liking.

Step #2: Populate the Breadboard

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  • Install the 555 timer on the breadboard, straddling the central "trench."
  • Add the resistors: 1.5kΩ (Brown-Green-Red-Gold), 47kΩ (Yellow-Violet-Orange-Gold), 15kΩ (Brown-Green-Orange-Gold), 2.2kΩ (Red-Red-Red-Gold), 10kΩ (Brown-Black-Orange-Gold)
  • Add the 1000μF and 1μF capacitors. These are polarized and need to go in the right way around.
  • Add the diode. This is also polarized and needs to go in the right way around.
  • The schematic here shows all of the connections.

Step #3: Connect the Offboard Components

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  • Wire up the servo's red (+5v), black (Gnd), and yellow (Control) connections from the circuit on the breadboard.
  • Connect the photoresistor (aka light-dependent resistor or LDR) to the circuit using wires long enough to reach up the wooden arm.
  • Drill a small hole in the end of the arm to hold the photodiode and insert it. Tape the wires down the arm to the breadboard.
  • Attach the wooden arm to the servomotor such that it leans slightly away from the ball. Mount the breadboard and servomotor onto the stand.
  • Connect the battery holder to the power and ground rails along the sides of the breadboard.
  • The connections you may not be able to see on the breadboard (second photo) are: Diode to Pin 6 of the IC, large cap between +5V and the 6th column of holes, small cap between Ground and Pin 2 of the IC.

Step #4: Final Setup

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  • I built the frame from what I had lying around. To hold the servo and breadboard, I glued down a few off-cuts of plywood.
  • For the arm, I used a 6" piece of square dowel. I wrapped the photoresistor wires around it and secured them with tape.
  • I used jeweler's wire to secure the arm to the servo — just loop the wire around and twist it tight.
  • Position a desk lamp so that it shines light towards the photosensor, from the other side of the hanging object.
  • Insert batteries into the battery holder and watch the fun. If the arm swings the wrong way, turn the servomotor around.
  • It might take some tweaking to find the "sweet" spot" where the light/sensor/ball all align to create the whacking action. But when you get it, it's pretty funny to watch...
  • You can see the ball whacker in action here.


This project is a great example of a cybernetic, or "self-governing" system. These kinds of systems have been around since the steam age (check out fly-ball governors for an early example of the technology).

With this kind of analog robotic control system. remarkably human-like behaviour can be obtained without the need for complex digital programming.

Steve Hobley

Steve Hobley

This week, I have been mostly working on...

I've been tinkering around with bits of technology since I was five years old. I used to take the telephone apart at home, just to see how it worked.

After a couple of years I could even put it back together again - and sometimes it would continue to work.

  • Steve Hobley

    I’m not familiar with a servo motor with only 2 wires. It would need power, signal and Gnd to function. Can you supply the make and model number?

  • This was a good project for a single afternoon with some kids. We used paint stirrers and an old piece of wood from my neighbor’s wood pile. Didn’t put the photo resistor onto the stick. I think we were so happy it worked…